September 2013 - kernelnewbies@kernelnewbies.org

How to get the inode - no path_lookup
by Rishi Agrawal Sept. 17, 2015

Sept. 17, 2015

Hi All, I had a module which used the path_lookup function to print the details of any file's inode. I now want to rewrite that module in order to show some juniors how to write some code in kernel. I am using 3.4.6 kernel, I tried finding out path_lookup but google showed that it has been removed. I tried the following code then which did not work . . . dentry = kern_path_create(AT_FDCWD, filename, &path, 1); if (IS_ERR(dentry)) { printk("Failed to obtain the dentry"); return; } its not returning dentry I again tried after seeing the implementation of vfs_stat function user_path_at(AT_FDCWD, filename, lookup_flags, &path); but this also fails. I am using a proc interface to pass the filename, and copying the filename into a kernel buffer. How can I get a copy of vfs inode for a file name. -- Regards, Rishi Agrawal

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RE: Debugging techniques inside kernel
by Anand Arumugam April 9, 2014

April 9, 2014

Try using strace and see if all of the different crashes have the same or similar looking call stack. Here's a link on how to use it in case you didn't know already: http://stackoverflow.com/questions/174942/how-to-use-strace -----Original Message----- From: manty kuma Sent: 7/7/2013 8:06 PM To: Valdis Kletnieks Cc: kernelnewbies(a)kernelnewbies.org Subject: Re: Debugging techniques inside kernel Hi Valdis, Thanks for the info. I have serial logs (using minicom). This is not helping me much as randomly memory is getting crashed and everytime it changes. Even the logs before the crash are varying everytime. On Mon, Jul 8, 2013 at 11:49 AM, <Valdis.Kletnieks(a)vt.edu> wrote: On Mon, 08 Jul 2013 11:22:16 +0900, manty kuma said: > For ex : *CONFIG_SLUB_DEBUG_ON, *CONFIG_DMA_API_DEBUG > I am just told to use these configs and check. Not sure how to use them > effectively. Please share if you are aware of these kind of configs and how > to use them. To have them at one place it would be useful for debugging > beginners like me. Most _DEBUG config options are set-and-forget - you turn them on and there's no further configuration needed. They enable additional code that does additional sanity checking and if it finds a problem, it issues a printk() or a stack dump or similar which shows up in dmesg. If you're chasing memory corruption, it may be useful to use netconsole or a serial console to send out all the printk() output in real time, as often an error will be detected but the system will panic()/crash/hang in a way that your local syslog daemon is unable to write the message to disk. (I'm told that on newer UEFI based systems, you can use pstore in a similar fashion, but have not tried it myself yet).

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simplefs - a ridiculosly simple file system from the scratch
by Sankar P April 1, 2014

April 1, 2014

Hi, For learning purposes, I thought I would implement a filesystem from the scratch. It has reached a state where I can do a 1.0 release. The sources are at: https://github.com/psankar/simplefs The layout is simple. The first (zeroth) block is the superblock and contains information like fs magic, number of inodes, freeblock map etc. The next block is the inode store. Creation of files and directories is supported. Nested directories are supported. A file can grow up to one block only. The data blocks for a directory contain the filename and inode number of its children. For files, it will obviously contain the actual data of the file. There are three rudimentary locks to make sure the accesses are thread safe. The 1.0 version is filled with a lot of TODOs and memory leaks. Also while implementing this, I realized that it is a bad idea to maintain the superblock and the freeblock store together in a block. So I am planning to change the on-disk layout in the next version. Apart from moving the super block to minimize locks, I am planning to implement support for extents in the next version, and journalling in the next-to-next version. The next version will take some time to come, though. But if someone wants to try filesystems from the scratch, I thought it may be useful if I share the link now itself. Also, I take this moment to thank the kernelnewbies list, especially the regular people like Mulyadi Santosa, Valdis Kletnieks, Rajat Sharma, Greg Freemyer etc. Also I would like to thank Neha Naik, Manish Katiyar who helped me with some queries during this particular implementation. I saw that Manish Katiyar has also done a similar from-the-scratch implementation and it motivated me to pursue further. Any feedback on the code is welcome. Although, I want to inform that I plan to change the locking until after I decide on a good on-disk layout for the next version, which will by-design minimize the locking needs, by splitting out freestore from superblock, keep track of children inode in a better way etc. Also I have been resisting the urge to look at ext or any other filesystem's design so as to not skew my thoughts and go with a fresh state of mind and get my own design :) Thanks. -- Sankar P http://psankar.blogspot.com

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mmap giving Permission Denied error
by Ravi Teja March 8, 2014

March 8, 2014

Hello all, I tried mmap'ing to address zero after setting */proc/sys/vm/mmap_min_addr*to * 0*, but mmap is giving Permission Denied error. My C code is as below - #include <stdio.h> #include <stdlib.h> #include <sys/mman.h> int main(void) { int *ptr = NULL; ptr = mmap(0,4096,PROT_READ|PROT_WRITE,MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED,-1,0); if(ptr == MAP_FAILED) { perror("Error in mapping\n"); exit(1); } printf("After mmap\n"); *ptr = 16; printf("Contents of address 0x%x is :: %d\n",ptr,*ptr); return 0; } Am I doing anything wrong here? Thank you in advance. Regards, Ravi Teja

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RE: Introducing Myself, Looking to Learn
by Varad Gautam Nov. 14, 2013

Nov. 14, 2013

---------------------------------------- > But please note that if you're > finding it *completely* incomprehensible, you're probably not going to > be able to make any real code contributions until you get up to speed. Hi Vladis! Thanks for replying. I think I would be fine with writing real code once I figure out what goes where. Is there something I should start with, which would help me bring myself at par? Thanks. Varad

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Any interesting linux projects?
by manty kuma Oct. 9, 2013

Oct. 9, 2013

I would like to work on some open source Linux projects. Please share some interesting projects that i can work from home. I am not looking for anything specific. Anything would do. Any interesting debugging tool or some new feature or that of fixing errors in bugzilla.. Please list anything. Only thing is i Know only C and Assembly(AVR and ARM). I know i am far from contributing to the mainline kernel. So want to start from something small where less competition is there to start with. I think the project we choose depends on our current skillset, so is there any site that kind of lists these projects and order it based on the skills? :) No i guess. But still any good sites are also welcome.

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can a bottom half be scheduled by another BH ??
by Vishwas Srivastava (vishsriv) Oct. 5, 2013

Oct. 5, 2013

Hi Group, I am a bit unclear if, I could schedule another BH while I am already running in a BH ? For example, if I am running in a SOFT IRQ, can I schedule a tasklet from there ?? Thanks, Vishwas S

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How to see the list of Interrupts that have occurred so far?
by manty kuma Oct. 1, 2013

Oct. 1, 2013

Hi, I would like to know how i can see which interrupt has occurred on an running maching using T32? I am debugging some cache errors, for which i wish to keep an eye on the interrupts. In short i want to know how does /proc/interrupts gets gets the list of the interrupts? Thanks, Manty

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anyone compiling busybox with "arm-eabi-" cross compile toolchain???
by sandeep kumar Oct. 1, 2013

Oct. 1, 2013

Hi All i am facing some build errors with busy box compilation with CROSS_COMPILE=arm-eabi- Busybox version: 1.21.1 Error: In file included from include/libbb.h:13:0, from include/busybox.h:8, from applets/applets.c:9: include/platform.h:138:20: fatal error: limits.h: No such file or directory Looks like i missed some include path..can anyone who tried this help me?? -- With regards, Sandeep Kumar Anantapalli,

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Finding a rogue IO source
by Phillip Susi Sept. 30, 2013

Sept. 30, 2013

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 I have an unused disk ( no mounted filesystems ) that I put to sleep ( hdparm -Y ) and every 10 minutes, the link is reset to process a request coming from a short lived process named "pool". I think it is a kernel work queue. I inserted a dump_stack() but it stops when it gets to the syscall and so doesn't trace back into what work item is running. How can I figure out what work item is doing this? -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.17 (MingW32) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEcBAEBAgAGBQJSQyVcAAoJEJrBOlT6nu75uqkH/j0zivSWfM5XsaDDJZ4gjb8i kvVlaidw9Wu2cv7lgVQB8Ml3eTmqx3ia9I72H9OWJcm324EJ10dD+3jiWAoT2T72 3uojSlbSrZ+0Yp+91EgKjc0/5D2Kz4vVI2rELHYt6ATA8rfLnA6Syrebr3zgY05m 41/pzBieT/lUBpQzWfkVxOtKk+Ma46uAmu1Ipo+G8wwFehbjT/LN57Bv3uFM4M7J sFaBAPCs8nNeAzR+Ud1Yhj6zNZnNct2bIm5oo7Lt7JPuBMzli8q+FVFLrvXOWU8M WXhkalBTYqlmNLXbNsJtYO2IfLFNlNsrYCoyS4IHKvHj8lSzlAKU/dO/OtbJFng= =/RfH -----END PGP SIGNATURE-----

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